The present invention relates to a device for monitoring a variable electric signal, as well as a component for constructing said device. Although not exclusively, the invention is particularly appropriate to the passivation of malfunctions of electric circuits on board aircraft.
For safety reasons, the orders for controlling at least certain elements of an aircraft are elaborated by at least two different devices. Thus, should there be divergence between the results given by these devices, the aircraft remains controllable by at least one of these devices during the time for confirming such divergence. If it exists beyond this confirmation time, the control system is disengaged. For this, the different signal elaboration devices may be disposed in parallel, and a signal selection system may be provided comprising a switch capable of connecting the output of one of these devices to said controlled element, under the dependence of comparators connected respectively between the outputs of said two devices. Such a selection systems with switch and comparators have the disadvantage of a certain inertia so that, if a breakdown of the device connected to the element to be controlled occurs, said element remains for a few moments connected to the malfunctioning device before being controlled by a device which has remained in operating condition. The result is a discontinuity of control which is prejudicial to piloting of the aircraft. In addition, such a system is likely to react to transitory phenomena so that it may inopportunely disconnect a working device to replace it by another; this may have disadvantages when a main signal elaboration device and auxiliary signal elaboration devices are provided and when it is desired for the element to preferably controlled by said main device.
Thus, to overcome the disadvantages of signal selection systems with switch and comparators, selection systems are preferably used, called voters, permanently receiving in parallel the outputs of said signal elaboration devices and delivering at their output that one of said signals which has the middle value with respect to the others. Thus, any discontinuity of control is avoided. However, in the case where the output signals of the signal elaboration devices are not strictly in synchronism, such a voter has the disadvantage of delivering an output signal (so controlling said controlled element) formed alternately of sections of several of said signals and not a single continuous signal chosen from the signals which are applied thereto. In addition, through their very operation, voters tend to favor the most delayed signal and so cause a delay in the control of said element. To avoid such drawbacks, faster computers could be used, but that would have other disadvantages, particularly related to weight and cost with consequential effects on the reliability.